A private plane with 10 U.S. passengers and two crew members died after their plane crashed in Costa Rica on Sunday, officials said.
Costa Rica's president, Luis Guillermo Solís, said an investigation would begin Monday into what caused the aircraft to go down shortly after 12 p.m (1 p.m. ET) in Guanacaste, a province in the country's northwestern corner popular with international tourists.
Guillermo Solís didn't identify the victims. He said the plane belonged to a Costa Rican airline, Nature Air.
In a brief interview with NBC News, Tamara Jacobson said that a brother, Bruce Steinberg, and four other family members were among the dead: Irene, Matthew, William and Zachary, Jacobson said.
"They were the most amazing family," Jacobson said, adding that the family recently celebrated the birthdays of three generations of relatives.
Former President Laura Chinchilla wrote on Twitter that Juan Manuel Retana, her cousin, was among the dead.
"You will remain in our beloved heart," she said.
Dawn Wolf, who owns property roughly 45 minutes from the crash site, said she was eating lunch with her family near the airport when she saw the plane fly overhead.
The aircraft had a single propeller, she said, adding that such planes are sometimes used to get from small towns to bigger cities for about $50.
"The plane was super low and then all of a sudden [it] veered to the left and crashed into the ground of the mountain sideways, wing first," she said.
The plane was in the air less than a minute.
Wolf heard a boom, then she said she saw an explosion 20 seconds later.
The "plane was broken into two," she said.
In a statement, the U.S. State Department confirmed the deaths of "multiple" Americans but declined to provide additional details.
"We express our condolences to all those affected by this tragedy," the statement said. "We are in contact with Costa Rican aviation authorities and will continue to monitor the situation."